1. Field of the Invention
This invention relates to a plasma display, and more particularly to a plasma display having high luminous efficiency and brightness.
2. Description of the Related Art
A plasma display displays a picture by utilizing visible lights emitted from phosphors when ultraviolet rays generated by gas discharge excite the phosphors. The plasma display has advantages in thinness, lightness, and realizing a high resolution and large-scale screen over a cathode ray tube (CRT) that has been the mainstream display mechanism for decades.
The plasma display is composed of a plurality of discharge cells that are arranged as a matrix. According to the supplied voltage for driving, the plasma display is largely classified into a direct current DC type and an alternate current AC type, and nowadays the AC type plasma display is mainly used.
Referring to FIG. 1, there is shown a related art plasma display of a 3-electrode AC surface discharge type. The plasma display includes a front plate 10 where a picture is displayed, and a rear plate 20 separated from the front plate 10 with designated gap. The front and rear plates 10 and 20 are coupled with a frit glass.
The front plate 10 includes: a common sustain electrode Z and a scan & sustain electrode Y which are arranged in a pair for maintaining discharge lights of discharge cells through the electric discharge therebetween; a dielectric layer 12 for insulating the common sustain electrode Z and the scan & sustain electrode Y and limiting the discharge current therebetween; and a protection layer 13 for preventing the damage of the dielectric layer and improving the emission efficiency of secondary electrons.
The common sustain electrode Z includes: a transparent electrode Za made from indium-tin-oxide (ITO); a bus electrode Zb made from metal; and a black layer 14 which are formed between the transparent electrode Za and the bus electrode Zb. The black layer 14 has electrical conductivity and is made from ruthenium-oxide, lead-oxide, carbon compound, or the like.
The scan & sustain electrode Y includes: an transparent electrode Ya made from ITO; a bus electrode Yb made from metal; and a black layer B which are formed between the transparent electrode Ya and the bus electrode Yb. The black layer B has electrical conductivity and is made from ruthenium-oxide, lead-oxide, carbon compound, or the like.
The rear plate 20 includes: address electrodes X crossing the common sustain electrode Z and the scan & sustain electrode Y; a dielectric layer 22 for insulating the address electrodes X; barrier ribs 21 formed on the dielectric layer 22 to partition discharge spaces, respectively;, and a phosphor layer 23 formed on the barrier rib 21 and the dielectric layer 22 to emit visible lights of one color among the red R, green G, and blue B colors by being excited and transited by ultraviolet rays.
Discharge gases with the pressure range of 300˜400 Torr are filled in the space between the front plate 10 and the rear plate 20. The discharge gases are mainly He, Xe, Ne, Ar, and the mixed gas thereof. Here, the Xe gas plays a role as the source of vacuum ultraviolet rays causing the phosphor layer 23 to emit visible lights, and the gases He, Ne, Ar and the like play a role as buffer gas.
This plasma display is mainly driven by the well-known Address and Display Separate (ADS) method in which the data writing period and the display period are separated in time.
In order to express gray levels of a picture, the plasma display is driven by frames, each of which is divided into several sub-fields having different emission frequencies with each other. Each sub-field is again divided into a reset period for uniform discharging, an address period for selecting discharge cells, and a sustain period for realizing the gray levels according to discharge frequencies. The address period corresponds to a data writing period, and the sustain period corresponds to a display period. For instance, when it is intended to display a picture of 256 gray levels, a frame interval of 1/60 second (i.e. 16.67 ms) is divided into 8 sub-fields. While the reset and address periods of each sub-field are identical for each sub-field, the sustain periods and the discharge frequencies are increased at the ratio of 2n (where n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field in accordance with the number of sustain pulses. Since the sustain period is different in each sub-field, it is possible to express a gray scale of a picture.
In the address period, when the potential difference between the scan & sustain and address electrodes Y and X reaches the range of 150˜300V, a writing discharge, i.e., an address discharge, occurs at the corresponding discharge cell, and so the wall charge is accumulated on the dielectric layer 12 of the discharge cell. When an alternate current is applied between the common sustain electrode Z and the scan & sustain electrode Y at the discharge cells selected by the address discharge, a sustain discharge occurs. Within these cells, electric fields generated by the sustain discharge accelerate electrons of the discharge gases. These accelerated electrons collide with neutral particles of the discharge gases. By these collisions the neutral particles are ionized into electrons and ions. This ionization process progresses in gradually higher rate in accordance with the growing number of collisions between the ionized electrons and the neutral particles of gases. This fast ionization process consequently transforms the discharge gases into plasma with emitting vacuum ultraviolet rays (VUV) in parallel. These vacuum ultraviolet rays excite the phosphor layer 23 to generate visible lights. The generated visible lights are radiated externally through the front plate 10, so the light emission of the discharge cells, displayed pictures, can be recognized externally.
In order to express gray levels of a picture, the plasma display is driven by a time divisional method wherein each of frames is divided into several sub-fields having different emission frequencies with each other. Each sub-field is again divided into a reset period for uniform discharging, an address period for selecting discharge cells, and a sustain period for realizing the gray levels in accordance with discharge frequencies. For instance, when it is intended to display a picture of 256 gray levels, a frame interval of 1/60 second (i.e. 16.67 ms) is divided into 8 sub-fields. While the reset and address periods of each sub-field are respectively identical, the sustain periods and corresponding discharge frequencies are increased at the ratio of 2n (where n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field in accordance with the number of the sustain pulses. Since the sustain period is different in each sub-field, it is possible to determine the brightness and the chrominance of displayed pictures through a combination of the sub-fields.
The related art plasma display, however, has the problem of low luminous efficiency due to the structure of electrodes. This problem is explained in detail with reference to FIGS. 3 and 4. Referring to FIGS. 3 and 4, the transparent electrodes Za and Ya of the respective common and scan & sustain electrodes Z and Y are made from ITO on the front plate 10 in order to reduce the degradation of the aperture ratio. These transparent electrodes Za and Ya are uniformly patterned to have the width of about 300 μm. The black layer 14 and the bus electrodes Zb and Yb are stacked on the transparent electrodes Za and Ya respectively. The bus electrodes Zb and Yb of the respective common and scan & sustain electrodes Z and Y are formed on the black layer 14 and made from Ag or Cr—Cu—Cr. The black layer 14 and the bus electrodes Zb and Yb are uniformly patterned to have narrower width than those of the transparent electrodes Za and Ya. Driving signals are applied to the transparent electrodes Za and Ya via the bus electrodes Zb and Yb respectively.
When the sustain voltage is applied to one of the common sustain electrode Z and the scan & sustain electrodes Y, due to the structure of the electrodes of the front plate as set forth above, the discharge begins at the place between the transparent electrodes Za and Ya having a small gap, and spreads out along the direction of the width of electrodes as shown in FIG. 4. These sustain discharges accelerate electrons of the discharge gases. These accelerated electrons collide with neutral particles of the discharge gases. By these collisions the neutral particles are ionized into electrons and ions. Similar collisions between the ionized electrons and the neutral particles of gases continue so as to transform the discharge gases into plasma and to emit vacuum ultraviolet rays (VUV) in parallel. During the excitation and transition of the discharge gases these vacuum ultraviolet rays generated with the direction of the arrows of FIG. 4 excite the phosphor layer 23 to emit visible lights.
In the related art plasma display, however, some of the vacuum ultraviolet rays generated by the sustain discharge vanish and cannot reach the phosphor layer 23. In other words, some of the vacuum ultraviolet rays generated from the inner electric fields between the common and scan & sustain electrodes Z and Y come to be ineffective ultraviolet rays, vanishing within the electric fields of the cell and not radiating toward the phosphor layer 23. These ineffective ultraviolet rays reduce the luminous efficiency of the plasma display and the brightness of displayed pictures, and raise the power consumption by increasing discharge voltage.